1. Field of the Invention
The present invention concerns with an improvement of a peak detector circuit designed for use in an FM (frequency modulation) measuring instrument or apparatus to measure frequency transitions.
2. Description of the Prior Art
For having a better understanding of the invention, the related prior art will be described in some detail by referring to FIG. 1 of the accompanying drawings which shows in a schematic circuit diagram a measuring system including an FM transmitter denoted generally by a reference numeral 10 and an FM measuring apparatus denoted by 20. The FM measuring apparatus 20 is composed of a local oscillator 20A, a converter or mixer 20B, a limiter 20C, an FM detector 20D, a level regulator 20E, a peak detector circuit 20F, an indicator 20G, a low frequency oscillator 20H and a second level regulator 20K connected in the manner as shown. As will be seen, the FM measuring apparatus 20 is a sort of FM receiver which is so arranged as to measure the modulation characteristics and others of the FM transmitter 10. To this end, the latter is connected to an FM modulation exciting source constituted by the low frequency oscillator 20H and the level regulator 20K incorporated in the FM measuring apparatus 20. On the other hand, the FM measuring apparatus 20 receives an FM signal from the FM transmitter 10, which signal is demodulated by the FM detector 20D. The peak detector circuit 20F detects the peak value of the demodulated signal, the detected demodulated signal being applied to the indicator 20G. In this way, the modulation frequency characteristics and the like of the FM transmitter 10 can be measured.
FIG. 2 shows a hitherto known circuit arrangement of the peak detector circuit 20F used in the system shown in FIG. 1. Referring to FIG. 2, a reference character 1A denotes an amplifier, 2A and 3a denote diodes, respectively, 4a denotes a capacitor, 5A denotes a resistor, 6A denotes an amplifier and 7 denotes an adder. Reference characters 1B to 6B denote same circuit elements as those designated by 1A to 6A, respectively. It should however be noted that the diodes 2B and 3B are connected with the polarities opposite to those of the diodes 2A and 3A, respectively. Consequently, the input signal to the peak levels circuit 20F shown in FIG. 2 is detected in respect to the peak levels of both negative and positive polarities. In this connection, it is to be mentioned that the input signal to the peak detector circuit shown in FIG. 2 may include a wide band signal such as a signal having an extended period and an impulse-like shot signal. In order to assure a high measuring accuracy even in such case, the charging time of the capacitor 4A is required to be shortened. To this end, the capacitor 4A is usually selected to have a small value, while the amplifier 6A connected to the capacitor 4A for holding a charging voltage across the capacitor is designed to exhibit a high input impedance.
FIG. 3 of the accompanying drawings shows waveform diagrams for illustrating relationship between the input signal and the output signal of the peak detector circuit 20F of the circuit configuration shown in FIG. 2. More specifically, the waveform of the input signal to the peak detector circuit is illustrated at (a), while that of the output signal thereof is shown at (b). As will be seen, the input signal (a) has peaks P and Q having greater instantaneous values relative to a steady level R. These peaks make appearance as peaks P' and Q' in the output signal waveform (b). A will be seen in the waveform (b), although the peak detector circuit can make rapid response to the rising edge of the peaks in the input signal (a), remarkable delay is involved in following the falling edge of the peak of the input signal to reach the steady level R'. This can be explained by the fact that the capacitor 4A is discharged trough the amplifier 6A having the high input impedance.
The time lag involved in the restoring of the output signal to the steady level R' in response to the falling edge of the peak in the input signal is accompanied with problems mentioned below.
When the frequency transition of the FM transmitter 10 is to be measured for the purpose of calibration or the like, the FM transmitter 10 is excited from the low frequency oscillator 20H through the level regulator 20K for controlling the FM modulation thereof. The FM signal produced by the FM transmitter 10 is received by the FM measuring apparatus 20 to be displayed by the indicator 20G. In this connection, it is noted that when the frequency transition or change of the FM signal is effected stepwise so as to step a desired value by consulting or observing the information given by the indicator 20G, a lot of time will be taken until coincidence is established between the desired value and the one display by the indicator 20G because of the significant time lag involved in the output signal of the peak detector circuit 20F in following up the decreasing in the level of the input signal thereto, as described above by referring to FIG. 3, which in turn means that the measurement is very time-consuming, giving rise to a problem.